The most important technological value of the high superconducting transition temperature superconductor Bi2Sr2CaCu2Ox (referred to herein as “Bi-2212”) may be as a round wire operated at “low temperatures”, i.e. 4.2K. That is because Bi-2212 is the only superconductor that can carry a significant supercurrent in the technologically useful form of a round wire in very high magnetic fields, i.e. above 23 Tesla (T). As high field uses inevitably involve construction of some form of coil, reliable Bi-2212 coil manufacture procedures are needed to maximize the potential of this material.
The coil fabrication technology used for the present high field superconductor material, Nb3Sn, is called the “wind-and-react” process, e.g., Taylor et al., “A Nb3Sn dipole magnet reacted after winding,” IEEE Trans. Magnetics Vol. MAG-21, No. 2, 1985, pp. 967-970. Typically a Nb3Sn precursor composite, either Nb filaments and Sn sources in a Cu matrix, or Nb filaments in a bronze matrix, is wiredrawn to a final diameter ˜1 mm and insulated with a glass yarn braid impregnated with a carbonaceous binder such as an organic resin. This wire is wound onto a coil former and heat-treated first to a temperature to burn off the carbonaceous binder, and then to the Nb3Sn formation temperature. This is typically done by burning the binder in air or oxygen at a relatively low temperature (˜300° C.) compared to the Nb3Sn reaction heat treatment temperature (˜650° C.). Any carbon that remains trapped within the windings after the binder is burned has no effect on the Nb3Sn phase formation.
It is very desirable to adopt this “wind-and-react” process for Bi-2212 coil fabrication, but in practice this has been difficult. The type of glass braid used for Nb3Sn coils fully melts at the reaction temperatures needed for Bi-2212 coils, so some combination of glass and ceramic, or pure ceramic is needed as the insulation material. Prior art Bi-2212 coils are plagued with many defects amongst the internal windings after reaction. The defects are often visually indicated by black stains (see Denis Markiewicz et al., “Perspective on a Superconducting 30 T/1.3 GHz NMR Spectrometer Magnet,” IEEE Trans. on Appl. Supercond., Vol 16, No. 2, 2006, pp. 1523-1526), and the defects result in coils delivering a fraction of the current they should be producing based on short sample testing. These coils are typically heat-treated in a furnace with continuous oxygen gas flow. The carbonaceous binder, known in the paper industry as “sizing,” is converted to CO2 during an initial low temperature heat treatment. The CO2 can be trapped in the tight winding pack, and even with a continuous flow of oxygen it is not possible to purge this trapped CO2 gas out of such a tightly wound pack. This presents a major problem, as the atmosphere adjacent to the wire surface is critical to the formation of the optimal phase of Bi-2212. The insulated wire is packed very densely into the coil former with the gas path in and out of coil pack only a series of many small orifices. It is very difficult to remove any unwanted gas, such as what might be produced from burning the binder, through such small orifices. A simple oxygen gas purge does not flush out the residual gas contaminants deep in the winding. One cause of a coil not carrying the expected current is the improper or incomplete formation of Bi-2212 due to contaminated atmosphere in even a small section of the coil during the reaction (high temperature) heat-treatment. Even if this only happens in a small section deep inside of the winding, the extracting and testing of the failed section from the coil is impractical as it may be only a short section of many thousands of meters.
One prior art investigator attempted to overcome this problem by using oxidized Hastelloy fibers as insulation material and a highly gapped weave, but the coil current was only 67% of the short sample (an uninsulated, uncoiled reference sample of the same wire) value. Watanbe, et al, “Ag-Sheated Bi2Sr2CaCu2O8 Square Wire Insulated with Oxidized Hastelloy Fiber Braid”, Advances in Cryo Engineering, Vol. 54, 2007, pp. 439-444. In addition, such a thin weave is not practical, in that such materials are both difficult to apply industrially and such wide gaps are highly susceptible to electrical shorting.